Aerobic biological treatment of waste waters consists in inducing the development of bacteria which, through a physicochemical action, retain and feed on the organic pollutants. For the biological treatment of urban and industrial effluents use is made in particular of packing devices forming static trickle surfaces over which the waters to be treated are distributed as well as possible.
The waters trickle over surfaces serving as supports for the purifying microorganisms. Aeration supplies the necessary oxygen. Degradation of the nutritive material consisting of the organic pollutants of the effluents occurs. This degradation corresponds chemically to a dehydrogenation, which is called aerobiosis or anaerobiosis depending on whether the hydrogen is fixed on the molecular oxygen superficially or on another acceptor in depth, when the biological film is sufficiently thick.
The mathematical relationship which takes into account the elimination of the BOD (biological oxygen demand) of waste water on a bacterial bed is written as follows: ##EQU1## where: Lf=BOD of the clarified water.
Lo=BOD of the feed water. PA0 Kt=coefficient depending on the temperature and nature of the material. PA0 Ss=specific surface of the material (m.sup.2 /m.sup.3). PA0 H=trickling height. PA0 Q=specific flow rate. PA0 n=constant. PA0 m=exponent&lt;1 determined by experiment.
Packing devices of plastics material for installations for the biological treatment of waste waters already exist, but they have various disadvantages:
Some are composed of multichannel tubular sections and have to be transported to the site in the form in which they will be installed, thus constituting a considerable volume in relation to the active surface. Transport is therefore a very important factor in the cost price, which thus becomes prohibitive for long distances, particularly for export.
The small section of these multichannel sections makes it necessary for them to be grouped together in open-work holders which, since they have to stay in place during use, must be made of a metal resistant to the corrosive media found in effluents and are therefore expensive.
Other devices are composed of elements of slight height, which are stacked one on the other. These elements are in particular in the form of assemblies of corrugated sheets separated by flat sheets defining channels between them. However, the channels of one element do not in most cases face the channels of the elements above and below them. There is consequently a considerable risk of clogging, which can be cleared only by very onerous dismantling and re-assembly.
In certain devices the channels situated facing beams or support members are necessarily obstructed. The entire surface of these channels then becomes inactive from top to bottom.